(a) Technical Field of the Invention
The present invention relates to a method of evaluating the power transmission ability of a frictional power transmission belt, and to a method of aiding the designing for a belt drive system.
(b) Prior Art
When evaluating the power transmission ability of a frictional power transmission belt, an important characteristic of the power transmission ability evaluation of the belt is a characteristic that indicates a relationship between the shaft load (initial belt tension) for the slip ratio of the belt and the loading torque. Concretely, when the slip ratio is reached at its allowable limit (usually, 1%), as the shaft load is decreased and the loading torque is increased, the belt power transmission ability is evaluated highly. As a power transmission ability curve represented by such an allowable slip ratio, there is one known in the art in which the shaft load is plotted on the abscissa while the ST factor by loading torque normalization is plotted on the ordinate. Based on such a curve, evaluation of the power transmission ability of the belt is conducted.
Reference is made to FIG. 4 which illustrates a layout for evaluation of the belt power transmission ability. A belt c is wound around a driving pulley a and a driven pulley b. A load W is applied, at different values, to the driven pulley b for the application of belt tension by varying the center distance between the pulleys a and b. Then, for each value of the load W, the slip ratio of the belt for the input torque is found to obtain a curve shown in FIG. 5. However, such a curve shows only the power transmission ability of the belt in the same layout, so that it does not have generality because the transmitted torque varies, for example, when there are variations in speed ratio and in pulley diameter.
To cope with the above, the abscissa is changed from indicating the input torque to indicating the ST factor, which provides an ST diagram as shown in FIG. 6. The ST factor is the effective tension of the belt per unit contact length. As FIG. 7 shows, when the belt c is wound around the pulley a(b) having a radius r (unit: m) at a contact angle θ (unit: radian) and its effective tension is represented by Te (=T1 (tight side tension)−T2 (slack side tension)), the ST factor is given by:ST=(Tq/r)/rθ=Tq/r2θ=(T1−T2)/rθ, where Tq is the transmitted torque (unit: N·m).
Then, referring to FIG. 6, intersections of the respective loads with the ST diagram are obtained at a given slip ratio (for example, at a slip ratio of 1% or 2%), thereby to find a relationship between the ST factor and the shaft load (unit: N) that is maintainable at any arbitrary slip ratio, which is shown in FIG. 8. This is called a power transmission ability curve (i.e., a power transmission ability diagram) which shows a power transmission ability characteristic inherent to the belt by a value of the ST factor transmittable at a given slip ratio for the shaft load.
However, in the foregoing conventional method, even when the same belt is employed, there is the problem that the power transmission ability curve varies depending on the layout (the pulley diameter and the speed ratio) relative to the pulleys. For example, the same belt is run to find power transmission ability curves at three different speed ratios (i.e., in a Low condition in which the pulley diameter of the driving pulley is smaller than that of the driven pulley, in a Mid condition in which the driving and driven pulleys have the same diameter, and in a Hi condition in which the diameter of the driving pulley is greater than that of the driven pulley). The power transmission ability curves found in these conditions are different, as shown in FIGS. 9–11. FIGS. 9–11 show the Low condition, the Mid condition, and the Hi condition, respectively. In FIGS. 9–11, the driving pulley is referenced as Dr and the driven pulley is referenced as Dn.
Therefore, it is necessary to introduce some correction factor for the designing of a transmission condition (service condition) in a belt drive system for a belt whose power transmission ability curve varies depending on the layout relative to pulleys.
An object of the present invention is to facilitate evaluation of the belt power transmission ability and design of the transmission condition in a belt drive system by finding a belt power transmission ability curve that does not depend on the layout relative to pulleys without having to introduce any correction factor.